This proposal describes a long-term study of immunocontraception of feral mares with porcine zonae pellucidae (PZP) and the effects on fertility and ovarian endocrine function. Twenty-six mares have already been immunized with PZP and fertility inhibition was successful. Ten of those mares have been given an annual PZP booster inoculation for three additional years, with greater than 90% fertility inhibition. Five of these 10 mares will be boosted annually, through 1994 (7 consecutive years of PZP treatment), 5 will be treated through 1992 (5 consecutive years of PZP treatment), and 10 additional mares will be immunized and boosted through 1994. This will permit the study of (1) animals immunized over 1,2,3,4,5,6,7 years, (2) animals immunized and permitted to return to fertility, and (3) a group of females which were in utero when their mothers were initially immunized. In addition to fertility assessment, normalcy of ovarian function will be determined by indirect measurements of plasma estradiol (E2) and progesterone (Po) during seasonal estrus. Urinary estrone conjugates (E1C) will be measured by enzyme immunoassay (EIA) in order to reflect plasma E2 concentrations. Urinary Po metabolites (iPdG) will be measured by EIA to reflect circulating Po. Specific ovarian dynamics to be monitored will include follicular phase E2 secretion, ovulation, luteal phase progesterone secretion, and luteolysis. The use of urinary steroid metabolites to monitor ovarian function will preclude the need to capture or handle horses.